I have a Dell latitude E6400, when I swich it on, it is says I need to an admnistrator password. Wha should I do?

The administrator password has been set and must be supplied to allow the unit to load. This could be a BIOS password or an operating system password. You did not specify which one, but it sounds like it is BIOS. Either is resettable with available software diagnostics, but usually is a depot solution for the not-so-technical. Hopefully you did not come into possession of a "hot" machine which the original owner password protected. There are a lot more of these "floating around" these days in the present economy! I see about 2 per month which wind up being stolen machines. Be careful! It is a crime to possess a stolen machine, even if you did not take it!!